Waveguide to coaxial &#34;l&#34; transition



pril 23, 1963 J. c. BORGHETTI 3,087,127

WAVEGUIDE T0 COAXIAL "L." TRANSITION Filed Jul 15, 1960 PR/Ol? ART INVENTOR. JOSEPH C. BORGHETTI BY 1 OM -1 PM ATTORNEYS United States Patent Ofiice 3,087,127 Patented Apr. 23, 1963 3,037,127 WAVEGUIDE TO COAXIAL L TRANSITION Joseph C. Borghetti, Southborough, Mass, assignor to Microwave Development Laboratories, Inc, Wellesley, Mass, a corporation of Massachusetts Filed July 15, 1960, Ser. No. 43,215 2 Claims. Cl. 333-34) This invention relates in general to apparatus for converting electromagnetic energy in one mode to a different mode and more particularly, the invention pertains to rectangular waveguide to coaxial line transition devices.

In microwave transmission lines it is frequently desirable and often necessary to change from waveguide to coaxial lines. Many components such as duplexers and antenna feed horns are more easily constructed of waveguide. Since many of the microwave generators have coaxial output terminals, it is then necessary to convert from the coaxial line to waveguide. It may also be necessary to place the coaxial line normal to the waveguide where it is required to transmit energy around a corner. It is also more convenient, at the longer wavelengths, to produce a symmetrical field for application to rotary joints by the use of the coaxial mode. The problem, generally, is to provide for a transition between the dominant caxial TEM-mode and the dominant TE mode in the rectangular guide. The problem is somewhat more complex where the transition is required to change the direction of transmission of the energy.

The invention resides in a joint providing a transition from rectangular Waveguide to a coaxial line which is arranged perpendicularly to the waveguide. Such joints are commonly known as L transitions.

The invention, in essence, resides in providing an impedance matching element which is relatively easy to manufacture because it has no complex surfaces whose profiles are difiicult to duplicate and cannot be checked for accuracy except by special equipment. In the invention, the impedance matching element is stepped from side to side, rather than being stepped in height as is customary in conventional transitions. The side-to-side stepping of the impedance matching element causes the susceptance of the waveguide to be altered to provide the desired match over a band of frequencies. In addition to being stepped from side to side, the impedance matching element has its narrowest step slopping in height, the slope presenting a planar surface. For ease of manufacture, the element which short circuits the rear of the rectangular waveguide is made integral with the impedance matching element. The short circuiting element is provided with a ledge which acts to position the impedance matching element in its proper position within the rectangular guide.

The invention can be better understood by a perusal of the following detailed description which is to be read in conjunction with the accompanying drawings wherein:

FIG. 1 depicts a conventional waveguide .to coaxial L transition;

FIG. 2 is a perspective view of the impedance matching member utilized in the invention;

FIG. 3 is a vertical sectional view of a preferred embodiment of the invention; and

FIG. 4 is a sectional view taken along the longer axis of FIG. 3.

Referring now to the conventional waveguide to coaxial L transition of FIG. 1, there is depicted a rectangular waveguide 1 having the outer conductor 2 of coaxial line secured to its upper broad wall. The rear of the waveguide is short circuited by a conductive plug 8. Abutting the plug 3 is an impedance matching member 4 having a flat plateau 5 and a ramp portion 6 sloping downwardly toward the open port of the rectangular waveguide. In order to obtain a good impedance match over a band of frequencies, it has been customary to alter the contour of the ramp until the desired match is obtained. While a good match is obtained by that technique, the contour of the ramp is often a complex shape which is difficult to duplicate. Where the object is to manufacture items having identical characteristics, it is much simpler to deal with geometric shapes such as rectangles, conics, cylinders, and circles rather than with compound curves. An important consideration in the manufacture of transitions is the ability to produce devices having identical characteristics. Experience has shown that the complex shaped matching member of FIG. 1 is expensive to manufacture, requires special equipment to check the accuracy of the complex surface, and results in an inordinate amount of rejects, i.e., parts which are dimensionally so inaccurate as to be unacceptable. It is an object of the invention to provide an impedance matching member having a geometric shape in place of the complex shaped impedance matching member 4 of FIG. 1. It will be noted in FIG. 1 that the center conductor 7 of the coaxial line has an enlarged conic portion 8 which terminates upon the plateau 5 in the rectangular waveguide. It is well known that the purpose of the enlarged conic portion is to provide a good impedance match to the coaxial line.

A preferred embodiment of the invention is depicted in FIGS. 2, 3, and 4. A rectangular waveguide 10 is shown having one port short circuited by a plug 12, the plug being an integral part of an impedance matching member 1 3. The upper broad wall of the rectangular waveguide 10 is provided with a circular aperture. A coaxial line is arranged normal to the waveguide with its outer conductor l4 concentric with the circular aperture. The outer conductor is joined to the upper broad wall, preferably by brazing or soldering. Since sharp edges are distinctly undesirable, the edge of the outer conductor 14 is rounded off so that it flares smoothly into the rectangular guide. Y

The impedance matching member 13 is shown in perspective in FIG. 2. That member, in effect, has three sections providing two side-to-side steps. The first section is a narrow elongate ramp 15 extending along the longitudinal axis of the lower broad wall of the waveguide. The ramp 15 has a constant slope and presents an almost feather edge to wave energy entering the port of the rectangular waveguide. The second section 16 and the third section 17 are of equal height, the second section 16 being wider than the ramp section 15, and the third section 17 being wider than the second section. The ramp 15 rises to the height of the second section and merges into the second section. The section 16 has a circular bore in it to accommodate the center conductor 18 of the coaxial line. The center conductor consists of two cylindrical portions 19 and 20 of dilferent diameters, the two cylindrical portions being connected by a truncated conic portion 21. The cylindrical portion 20 is of the same height as section 16 and fits within the circular bore in section 16 so that the entire conic section protrudes above the surface of that section and the upper cylindrical portion 19 is within the outer conductor 14 of the coaxial line. The plug 12 is an integral part of the member 13 and is dimensioned to slide snugly within the rectangular waveguide until the lip 22 of the plug abuts the waveguides end. Since the lip 22 determines the extent to which the plug enters the waveguide, the lip is used to position the impedance matching member 13 so that the center conductor 18 is disposed concentrically with the outer conductor 14 0f the coaxial line.

The impedance matching member is secured to the bottom broad wall of the rectangular waveguide by brazing 3, or soldering and the center conductor 18 is similarly secured in the circular bore of section 16.

It may be observed from FIGS. 2, 3, and 4 that most of the sharp edges of the impedance matching sections of member 13 have been rounded off; This is done to prevent electrical breakdown within the transition due to the concentration of the electric field at sharp corners. The impedance matching member 13 is essentially an assemblage of rectangular shapes with the surfaces being planar and hence thatmember is relatively easy'to duplicate with a high degree of accuracy and can be checked for dimensional accuracy with standard measuring instruments. The side-to-side stepping of the impedance matching member causes the susceptance of the Wave guide to be altered to provide the desired match over a band of frequencies; It is evident that the number of steps may be altered as circumstances require and that the slope of the ramp maybe made less or more steep. It is alsopossible to change the ramp from a structure having a constant-slope to a structure which is stepped in height.

The dimensions of the transition joint are critical, if the joint is to perform properly at the intended frequency of operation. It-is possible, by adhering to certain principles of similarity, to make use of the dimensions of an existing device in the design of a like device to operate at a different frequency. For the principles involved, the readeris referred to pages 87 to 90 in volume 9 of the Radiation Laboratory Series, entitled Microwave Trans mission Circuits, published by McGraw-Hill; The dimensions given below (in inches) are for a waveguide to coaxial L transition intended to operate in the frequency range of 8.5 to 9.6 kilomegacycles.

While a preferred embodiment of the invention has been illustrated in the drawings, it is evident that modifications-can be made which do not depart from the essence of the invention. It is therefore intended that the invention not be limited to the precise structure illustrated, but rather that the scope of the invention be delimited by the appended claims.

I claim:

1. A waveguide to coaxial L transition comprising a rectangular wave guide having a circular aperture in one of its broad walls, a coaxial line extending normal to the waveguide, the coaxial line having its outer conductor concentric with the circular aperture and joined to the apertured broad wall, an impedance matching member in the rectangular waveguide secured to the other broad wall thereof, the impedance matching member having three sections of different widths, the first section being a narrow elongate ramp of constant slope, the second and third sections being of the same height, and the center conductor of the coaxial line extending through the circular aperture and having a conical portion joined to the second section of the impedance matching member.

2; In a waveguide to coaxial L transition of the type comprising a rectangular waveguide having one end terminated in a short circuit, the rectangular waveguide having an aperture in one of its broad walls through which passes the center conductor of a coaxial line, the outer conductor of the coaxial line being secured to the apertured broad wall, the improvement of an impedance matching member in the rectangular waveguide, the matching member being secured to the other broad wall of the waveguide, the matching member having a plurality of contiguous sections, each section being wider than its preceding section, the broadest section being closest to the short circui-ted end of the rectangular waveguide, the narrowest section being elongate and gradually descending in height from the level of the contiguous section toward the level of said other broad wall, all the sections except the narrowest having the same height, and the center conductor of the coaxial line extending into contact with an intermediate section of the impedance matching member.

References Cited in the file of this patent UNITED STATES PATENTS 2,659,055 Cohn Nov. 10, 1953 

1. A WAVEGUIDE TO COAXIAL L TRANSITION COMPRISING A RECTANGULAR WAVE GUIDE HAVING A CIRCULAR APERTURE IN ONE OF ITS BROAD WALLS, A COAXIAL LINE EXTENDING NORMAL TO THE WAVEGUIDE, THE COAXIAL LINE HAVING ITS OUTER CONDUCTOR CONCENTRIC WITH THE CIRCULAR APERTURE AND JOINED TO THE APERTURED BROAD WALL, AN IMPEDANCE MATCHING MEMBER IN THE RECTANGULAR WAVEGUIDE SECURED TO THE OTHER BROAD WALL THEREOF, THE IMPEDANCE MATCHING MEMBER HAVING THREE SECTIONS OF DIFFERENT WIDTHS, THE FIRST SECTION BEING A NARROW ELONGATE RAMP OF CONSTANT SLOPE, THE SECOND AND THIRD SECTIONS BEING OF THE SAME HEIGHT, AND THE CENTER CONDUCTOR OF THE COAXIAL LINE EXTENDING THROUGH THE CIRCULAR APERTURE AND HAVING A CONICAL PORTION JOINED TO THE SECOND SECTION OF THE IMPEDANCE MATCHING MEMBER. 